The manufacture of steel using an electric arc furnace (EAF) is a highly advantageous process in the modern steel industry. A drawback in the EAF manufacture of steel is the production of EAF dust waste by-products. The EAF dust is generated during the steel making process by a variety of mechanisms, including droplet ejection from the turbulent melt and vaporization. The vaporization mechanism is primarily responsible for the relatively high proportion of the non-ferrous metals in the dust such as zinc, lead and cadmium, which are vapors at a temperature of about 1600.degree. centigrade (2,912.degree. fahrenheit) in the EAF hearth. The droplets and vapors of vaporized non-ferrous metals are carried in the exhaust gas stream from the furnace along with mechanically entrained iron oxides, lime, etc. The vaporized metals condense as oxides and ferrites and are collected downstream in a baghouse and/or electrostatic precipitator. The production of 1 ton of steel will generate approximately 34 pounds (15.4 kg) of EAF dust as a by-product.
The rapid growth of the EAF steel process has made EAF dust one of the fastest growing and largest environmental problems worldwide. At present, there are approximately 600,000 metric tons of this hazardous waste generated annually in the USA and an additional 600,000 metric tons generated annually in the rest of the world. EAF produced steel is currently forty percent (40%) of total US steel production. It is expected to become the major source of steel produced in the USA early in the next century.
There is also a similar but lower level contaminated dust which is derived from the other major process for steel manufacturing, the basic oxygen furnace (BOF) . The BOF can be either top or bottom blown. Because the levels of toxic metals such as cadmium, lead and zinc are lower, BOF dust is not currently classified by EPA as hazardous.
BOF dust may be considered hazardous in the future and its non-iron contaminants make it difficult to impossible to utilize in current steel manufacture resulting in substantial "stock piles" worldwide.
BOF dust can also be processed by the present invention to recover both the ferrous and non-ferrous values contained therein.
Many other metallurgical dusts also contain metal values which can be recovered and environmental concerns which can benefit from the application of this invention.
Currently EAF dust is disposed of by pyro metallurgical processing, land-filling, stabilization and to a small extent, fertilizer production.
Pyro processing which represents the substantial proportion of all EAF dust treated, costs steel producers from US $150 to US $200 per metric ton. Assuming an average cost of US $175 per metric ton of EAF dust, the annual cash burden on this major essential industry is US $105,000,000 per year. This figure, however, greatly undervalues the true cost to the steel industry, to the public-at-large, and to our descendants from damage to the environment.
None of the known processes in practice totally eliminate economic and environmental waste.
Table 1 gives the average values of the EAF dust constituents for a sample plant for 1992 and 1993. Most notable are the high concentrations of iron (approximately 35%), zinc (approximately 20-23%), lead (approximately 1%), manganese (approximately 3.5%), and magnesium (approximately 2%). The nonferrous values represent potentially rich sources of metal values, such as zinc, lead, copper, cadmium, calcium and magnesium. The iron can be recycled back into the EAF furnace.
TABLE 1 ______________________________________ SAMPLE PLANT EAF DUST CONSTITUENTS FOR 1992/1993 ______________________________________ % Zn % Al % Pb % Fe % Cd % Cu % Mn % Na % Ba ______________________________________ Avg. 20.3 0.27 1.27 36.0 0.02 0.16 3.54 0.59 0.01 1992 Avg. 22.7 0.30 1.04 34.8 0.01 0.13 3.60 0.70 0.01 1993 ______________________________________ % CaO % Cr % Mg % Ni % V % As % SiO2 % Cl ______________________________________ Avg. 5.51 0.20 2.06 0.02 0.01 .0036 2.52 0.96 1992 Avg. 5.48 0.20 2.48 0.13 0.02 .0029 4.74 0.78 1993 ______________________________________
There are several problems associated with the disposal or processing of EAF dust. The high level of zinc in the dust is expected by many experts to cause problems with zinc build-up in a furnace if the dust is recycled directly to the furnace. High zinc vapors in the EAF furnace are known to cause shorting of the electrodes causing the electrical energy to be wasted instead of being used for making steel. A high proportion of the zinc in EAF dust is present in the form of ferrites (ZnO.Fe.sub.2 O.sub.3,) which have proven resistant to presently known leaching processes.
Currently, the steelmaker disposes of the EAF dust by shipping it to a company specializing in waste management, which charges a processing fee of up to $200 per ton of dust. In addition to the processing fee, there are freight costs and substantial potential liability in the over-the-road shipment of this hazardous waste.
The production of 1 ton of steel is generally expected to generate about 34 lbs. (15.4 kg) of EAF dust as a by-product. Therefore, a 100 ton hearth will produce about 3400 lbs. (1542 kg) of EAF dust. A typical EAF hearth produces over 10,000 tons of EAF dust per annum. When a $175 per ton cost is considered, the dust disposal cost for this typical plant is over $1,750,000 per year. Many of the more substantial EAF Steel companies have two, three or four hearths at a single steel complex. Their EAF costs rise proportionally.